Many types of clutches, particularly those used in power trains driven by very powerful engines, for example, marine propeller and bow thruster drives, can be damaged by engaging or disengaging the clutch (or clutches) at other than speeds close to idle. The possibility of damage is especially great when full engine speed is commanded and a reversible type clutch system is to be changed from engagement to drive the output in one direction to engagement to drive the output in the other direction. For example, assume that a bow thruster is commanded to go from full speed port to full speed starboard. The movement of the engine-clutch controller from full engine speed and port clutch to full engine speed and starboard clutch initiates a control sequence which is supposed to reduce the engine speed to idle, to cause the port clutch to disengage and the starboard clutch to engage and then accelerate the engine back to full speed.
Presently known systems are either pneumatic or hydraulic or a combination of both and rely on timing of the sequence of operations in that it is assumed that at the end of a certain time duration a particular step in the sequence has taken place. Generally, the time delays between the various operations are obtained by using long coils of tubing (or other storage volume) and throttling orifices to delay the conduction of pneumatic or hydraulic signals to the controlled elements. Among the disadvantages of presently known systems are the need for long time delays to be built into the systems to ensure that the operations inferred to have occurred within a certain time duration have in fact occurred. Notwithstanding the provision of large time delays, there is no certainty that an operation that was to have occurred within a certain time did in fact occur, and the next operation in the time control sequence may take place based on the false assumption that a previously required operation had occurred. For example, the engine speed may not have in fact dropped to idle for one reason or another, but the change from one clutch to the other will take place, because, apart from time, initiation of the clutch change is controlled without regard to engine speed. The commonly used coils of tubing are physically cumbersome and often difficult to maintain in good condition, and the time delay orifices are susceptible to dirt contamination and consequent malfunction.